Geoff, sorry to say that I'm burnt by the same issue. I'm still searching for a solution also.
My problem started around the week of 18/1/2021, and as I've made no changes to my home network I have to assume that a Windows update was applied in the background around then. Symptom is that File Explorer takes a long time to look around on start-up, and I can no longer connect to mapped drives (to my Netgear NAS) but I can open local drives.
I tried numerous things, after checking nbtstat, arp and ipconfig data, including clearing all internal caches I can find. I can ping the NAS by both it's IP address and name fine, but clicking on the mapped drive I now receive (eventually) an error stating either that the drive mapping is in use or that the path can't be found. Trying to open the NAS (under Network) in the left pane of File Explorer no longer shows the shares on the NAS - File Explorer just times out saying it can't access \<nas>.
I've also tried listing the fixed IP address of the NAS with its name in the hosts file, with no success.
And I can't even reload key Windows system files on my laptop as the "reset PC" option doesn't work either - it keeps failing without further information.
Interesting to note that I have a NVidia Shield media streamer on my network, and it can access the same drives on my NAS via SMB (SMB 2 or 3, not 1) to play content from there. It's clearly an issue isolated with the Windows PC, and not the NAS nor the local network.
I'll keep watching this thread, but I've run out of things I can think of trying sort of re-installing Windows - and I reeeeeally don't want to do that.